Current techniques for fabrication of a resistive film include thick film formation and thin film formation. The thin film formation is a process of forming a resistive film on an insulating substrate by deposition or sputtering in a vacuum chamber. This process is capable of forming a thin and uniform resistive film but requires large-scale production equipment, resulting in high cost.
The thick film formation is a process consisting of coating or printing a resistive film-forming paste on an insulating substrate followed by calcining. This process achieves high productivity with inexpensive equipment and at low cost, but the resistive films obtained by most of the frequently employed thick film formation techniques generally have a large film thickness and accordingly have a great heat capacity. Moreover, comprised of a sintered body of particles having an particle size of micron-meter order, the resulting resistive film not only suffers from large variation in resistivity but has poor strength in electric field. A thermal head using such a resistive film as a heating element involves a large energy consumption and exhibits poor thermal response and easily changeable resistivity.
A number of proposals have hitherto been made for the production of a thin resistive film by exploiting the thick film formation technique with inexpensive production equipment. One of these proposals is a metallo-organic deposition (MOD) process, in which a solution containing an organometallic compound is coated on a substrate, followed by calcining to decompose the compound to form a thin film comprising the corresponding metal oxide, etc. Reference can be made in JP-A-64-54710, JP-A-1-286401, and JP-A-1-220402 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). While an organic iridium compound is commercially available in the form of metal resinate for a resistive film-forming composition in the MOD process, the thin resistive film obtained by the MOD process using the known metal resinate of iridium compound still have unsatisfactory uniformity of film thickness, surface defects such as cracks or pinholes, and insufficient adhesion to the substrate. In addition, the organic iridium compounds known applicable to the MOD process are limited and expensive.